meaning of running throughout a situation

[Learning_English]

thread = a theme or characteristic running throughout a situation or piece of writing

Please explain this.
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Please let me know if I have made any mistakes in any of my sentences.

[Conchita]

The Cambridge Advanced Learner's Dictionary usually provides simple, easy-to-understand definitions. Here's what it says about the meaning of 'thread' you're looking for:

3 [C] The thread of a book, discussion, speech etc. is its story or the way that it develops, one part connecting with another:

One of the main threads of the film is the development of the relationship between the boy and his uncle.

Unfortunately my attention wandered for a moment and I lost the thread of (= forgot) what I was saying.

http://dictionary.cambridge.org/results.asp?searchword=thread&x=0&y=0